ISO Week (ISO 8601 Week)
An ISO week is a week as defined by the ISO 8601 date standard. Every week starts on Monday, and week 1 of a year is the week containing that year's first Thursday - equivalently, the week that holds January 4.
The rule
Because week 1 is anchored to a Thursday rather than to January 1, the first days of January can belong to the last week of the previous year, and late December can belong to week 1 of the next. Every ISO week has exactly seven days, Monday to Sunday.
Week-date format
ISO 8601 writes a week as a week date: the year, the letter W, and the two-digit week number. 2026-W31 is the 31st ISO week of 2026. A weekday can be added as 2026-W31-3, where the last digit is the day, counting Monday as 1 through Sunday as 7.
52 or 53 weeks
A year has 53 ISO weeks when January 1 is a Thursday, or a Wednesday in a leap year; otherwise it has 52.
How Sandtime.io fits
Sprints, shift rotations, and European business calendars run on ISO weeks, and Sandtime.io reports the hours worked in each one. To look up a week, use the ISO week number tool.
Related Terms
An ISO week is the standard form behind a week number, it often defines the reference period used for scheduling, and it frames how a weekly rest period falls across working days.